Drag background images in 3D view?
To the best of my knowledge, such functionality is not available out of the box. However, one other way would be to use an Empty with the Display type set to Image. Advantages are that you can move it just as you would any other object and adjust the opacity etc. Another advantage is that it displays in both Orthographic and Perspective view.



If you find it hindering you.. you can always just disable Viewport selection from the Outliner by clicking the arrow.